Crosslinking of the methine chains in methine compounds is a well known technique for increasing solution stability.
A detailed description of the conventional technique of crosslinked methine compounds is given in comparison with the technique of the present invention hereinafter in the "Detailed Description of the Invention" section.
Furthermore, the technique of adding sensitizing dyes to silver halide emulsions when manufacturing silver halide light-sensitive materials to extend the light-sensitive wavelength of the silver halide emulsion and to provide optical sensitization has long been known.
Many compounds have long been known as spectrally sensitizing dyes which can be used for this purpose, and examples of such compounds include the cyanine dyes, merocyanine dyes and xanthene dyes etc. disclosed on pages 198-228 of The Theory of the Photographic Process (third edition) by T. H. James (1966, Macmillan, New York).
These sensitizing dyes must not only extend the light-sensitive wavelength region of the silver halide emulsions but must also satisfy the various conditions indicated below if they are to be used generally in silver halide emulsions.
(1) They must have an appropriate spectral sensitization region.
(2) They must have a good sensitizing efficiency and enable sufficiently high sensitivities to be obtained.
(3) They must not give rise to fogging.
(4) The variation in sensitivity due to fluctuations in temperature at the time of exposure must be small.
(5) There must be no adverse interaction with the various additives, such as stabilizers, anti-fogging agents, coating aids and color formers which are being used.
(6) There must be no change in sensitivity on storing a silver halide emulsion which contains the sensitizing dye. In particular, there must be no change in sensitivity on storage under conditions of high temperature and high humidity.
(7) There must be no diffusion of the sensitizing dye which has been added to other light-sensitive layers and no color turbidity (color mixing) after development processing.
The conditions outlined above are of great significance when preparing silver halide emulsions for silver halide color photographic materials.
However, although various attempts have been made to prevent it, the fall in sensitivity on storing raw sample has not been prevented to a satisfactory degree.
In particular, an adequate performance in respect of the loss of sensitivity on storing raw sample cannot be obtained when polymethine dyes which have an oxidation potential of 0.60 (V vs SCE) or lower are used as sensitizing dyes.